A semiconductor wafer of, for example, silicon or gallium arsenide is produced in the form of a large diameter. This wafer is cut and separated (diced) into small element chips and is subjected to the subsequent bonding step and mounting step in which the chips are mounted on lead frames. In this process, the semiconductor wafer undergoes dicing, cleaning, drying, expanding and pickup steps in the state of being attached to a pressure sensitive adhesive sheet and transferred to the subsequent mounting step in which the chips are mounted on lead frames.
Adhesive sheets which are intended for use in the processing steps of wafers, from the dicing step up to the pickup step, are desired to have an adhesive force sufficient to retain wafer and chips thereon during the dicing step, but in the pickup step, they are desired to only retain an adhesive force of such an extent that no adhesive remains on the picked-up chips.
In the efforts for obtaining such a desired pressure sensitive adhesive sheet, Japanese Patent Laid-open Publication Nos. 60(1985)-196956 and 60(1985)-223139 proposed pressure sensitive adhesive sheets each comprising a substrate coated with a pressure sensitive adhesive comprising a low molecular weight compound having at least two photopolymerizable carbon-carbon double bonds in each molecule thereof which can be converted to a three-dimensional netted structure by light irradiation. These proposals are based on the concept that, in a pressure sensitive adhesive tape comprising a radiation transmittable substrate and, applied thereon, a radiation curable pressure sensitive adhesive, the radiation curable compound contained in the pressure sensitive adhesive is cured by irradiation so that the pressure sensitive adhesive comes to have a three-dimensional netted structure to thereby conspicuously lower the fluidity thereof.
The above-mentioned conventional pressure sensitive adhesive sheets have the following particular problems in the expanding and pickup steps.
The expanding step enlarges diced small element piece (chip) spacings to thereby facilitate the pickup of the chips. When the conventional wafer bonding pressure sensitive adhesive sheet capable of minimizing the amount of adhesive residue is used, the curing of the pressure sensitive adhesive layer is advanced to such an extent that the pressure sensitive adhesive layer becomes conspicuously hard. Thus, the sheet extensibility (expansibility) is decreased, thereby causing the attainment of desired chip spacings to be difficult. Illustratively, it occurred that the spacings between chips neighboring each other were not satisfactory to thereby cause recognition failure at the pickup step with the result that operation error was invited.
Further, in the above conventional pressure sensitive adhesive tape, the pressure sensitive adhesive layer becomes so hard as to result in deteriorated chip holding capability. Thus, it occurred that chips were dislocated at the time of pickup to thereby cause pickup failure.
Japanese Patent Laid-open Publication No. 5(1993)-214298 teaches a pressure sensitive adhesive sheet provided with a pressure sensitive adhesive layer having, added thereto, a polyether compound of relatively low molecular weight and an unsaturated oligomer of relatively low molecular weight for rendering the adhesive curable by radiation irradiation. However, when the amount of added low molecular weight component is too large, this pressure sensitive adhesive suffers from lowering of its initial adhesive strength, so that chip scattering would occur at the time of dicing. On the other hand, when the amount of added low molecular weight component is too small, the drawback would be encountered that the reduction of the adhesive strength is unsatisfactory upon radiation irradiation or the amount of adhesive residue is unfavorably large. Therefore, the control of adherent properties was very difficult before and after the radiation irradiation.